1. Field of the Invention
The present invention relates to a display device which holds a display image for a certain period of time even when the display device is out of communication range of wireless communication.
2. Description of the Related Art
In recent years, an individual identification technique using wireless communication with an electromagnetic field, an electric wave, or the like has attracted attention. In particular, an individual identification technique using an RFID (radio frequency identification) tag as a semiconductor device which updates data by wireless communication has attracted attention. An RFID tag (hereinafter, simply referred to as an RFID) is also called an IC (integrated circuit) tag, an IC chip, an RF tag, a wireless tag, or an electronic tag. The individual identification technique using an RFID has come to help production, management, or the like of an individual object, and is studied to be applied to individual authentication.
There is a passive type RFID which can transmit an electric wave or an electromagnetic wave each including information and which is driven by utilizing electric power of an electric wave or an electromagnetic wave (carrier wave) from an external portion (see Patent Document 1: Japanese Published Patent Application No. 2006-503376). In the passive type RFID, a power source for driving an RFID is generated by utilizing electric power of an electric wave or an electromagnetic wave (carrier wave) from an external portion, and thus, a structure without a battery is realized.
Meanwhile, a display device is actively developed as a large-sized display device such as a liquid crystal television becomes common. Further, as an application of a passive type RFID, a display device which directly transmits an image signal by wireless communication has also attracted attention (see Patent Document 2: Japanese Published Patent Application No. 2006-018132). It is needed that a display device is within a distance that can communicate with a wireless communication device. Therefore, electric power cannot be received when communication range with the wireless communication device is not suitable. Accordingly, there is a limitation that the display device is always within a communicable distance.
As for the RFID tag, the longer the distance through which an electric wave or an electromagnetic wave (carrier wave) transmitted from a wireless communication device is received, the more a supplied electric wave is attenuated and electric power which can be generated is reduced. Further, when the RFID tag is out of communication range, an electric wave itself is not supplied and electric power cannot be generated.
Further, there is a case where a communication distance between the RFID tag and the wireless communication device is desired to be long depending on a use. Therefore, a problem on receiving power in the case of long-distance communication cannot be ignored.
Meanwhile, operation of the display device can be controlled through an FPC or the like, using a driver circuit, a controller IC, and so on, which are formed of transistors. Thus, the degree of freedom is limited due to the shape of a wire cable such as an FPC.
The display device receives an image signal and generates electric power by wireless communication, and accordingly, it is necessary to be always within a communicable distance to hold a display image. When the display device is moved out of communication range with the wireless communication device, supply of electric power is stopped, and the display image is off.